1. Field of the Invention
The present invention relates generally to a method and apparatus for rock bolting a roof or a hanging wall in a mine, shaft, tunnel, or generally in any cavity in the rock, by means of bolts which are fastened in boreholes.
2. Description of the Related Art
The present invention relates to an improved embodiment of an anchor device for securing a rock bolt in a borehole, and as such represents an improvement to my previous U.S. Pat. No. 4,820,095, issued on Apr. 11, 1989. Basically, the device previously patented comprises an elongated cage having a perforated elongated wall confining an elongated chamber containing a bonding means for bonding the rock bolt within the borehole, the cage having a trailing end adapted to mount the anchor device onto the leading end of a rock bolt for insertion into the borehole.
FIGS. 1-6 of the patent have been reproduced herein, along with the following description of the figures.
Referring to FIG. 1, there is shown a first embodiment of the anchor device of the present invention comprising an elongated cage 1 having an elongated wall member 2 containing rectangular perforations 3 therein. Preferably, elongated wall 2 is cylindrical in shape. The elongated wall 2 confines an elongated chamber 4 which contains a bonding means such as a cartridge 5 containing a bonding composition 6. The bonding composition 6 may comprise a cement or mortar for bonding a rock bolt firmly within a borehole, but a preferred embodiment of the bonding composition is a two component chemical system comprising a resin and a hardener.
Such resins and hardeners are well known in the art. The hardener is a crosslinking agent, which upon contact and thorough intermixing with the resin will cause the resin to gel by crosslinking and thereby set-up to form a very hard bonding agent. In order to keep the hardener and the resin isolated until the moment of use, the two components are kept apart by frangible partitioning means. The partitioning means may be a frangible microencapsulation coating which may cover either one or both of the components in a plurality of intermixed microspheres. The components remain isolated from each other until the rock bolt penetrates the cartridge and crushes the microspheres. Alternatively, the partitioning means may be a plurality of elongated sheaths or bags of frangible plastic which confine the hardener and the resin separately so that they are not in contact with each other until the bags are ruptured by the rock bolt. A third method of keeping the two components from contact with each other is to provide a frangible partition member within the cartridge as an internal element thereof, which fractures as the rock bolt enters the cartridge. Since these partitioning means are all frangible, when the cartridge is ruptured, the partitioning means, not shown in FIGS. 1-6, will either tear, rupture, or fracture when the rock bolt penetrates into the cartridge, and the hardener and resin will become intermixed and react as the rock bolt passes through the length of the cartridge, the rock bolt generally being rotated to enhance mixing.
The elongated cage 1 has a leading end 7 which enters the borehole first and a trailing end which enters the borehole last. The trailing end comprises a neck down transition section 9 fixed to an elongated section 8. The bottom of resin cartridge 5 rests in the top of this neck down section 9. This neck down section 9 is in the shape of a truncated cone and the elongated section 8 is preferably cylindrical. A rock bolt 13 is forced into a central bore 11 which is contained within the elongated section 8 so that there is a tight compression fit of the rock bolt within the bore 11. The bore 11 is in open communication with the chamber 4 so that the rock bolt may eventually contact and penetrate the resin cartridge 5. In order to facilitate the penetration of the rock bolt within the bore 11 there is optionally provided at least one elongated slot 12.
The rock bolt 13 commonly will have surface ridges or teeth 14 on its outer surface which are adapted to provide an enhanced bonding surface for contact with the bonding agent which sets up in the borehole. As shown in FIG. 1, these ridges or teeth on the surface are in the configuration of a Chevron which is oriented with the apex of the Chevron pointed toward leading end 7 so as to resist the pulling of the rock bolt from the interior of the cage and from the bonding agent when the bonding agent has hardened. It is to be noted that the tight compression fit of rock bolt 13 within elongated section 8 prevents premature puncture of the resin cartridge by the rock bolt as the assembly is being pushed into the borehole.
FIG. 2 is a sectional view of the elongated cage 1 of FIG. 1 taken along line A--A. In FIG. 2, there is shown the elongated wall member 2 containing perforations 3, and confined therein is the cartridge 5 containing the bonding composition 6.
FIG. 3 is a sectional view of the elongated cage of FIG. 1 taken along sectional line B--B. There is shown in FIG. 3 the elongated section 8 at the trailing end of the cage 1 and confined therein is the rock bolt 13. Also shown is a slot 12 which allows the elongated section 8 to expand as the rock bolt enters the bore 11 of section 8.
An alternative embodiment as previously described in U.S. Pat. No. 4,820,095 is illustrated in FIG. 4, in which a cage 16 has an elongated wall 17 containing circular perforations 1. Confined within the chamber of the cage 16 is a bonding means such as cartridge 5 containing a bonding composition 6. As seen in FIG. 4, this embodiment is a substantially tubular elongated cage having a leading end 19 and a trailing end 21, with no neck down portion at the trailing end. The cartridge 5 is supported within the chamber by a pin 23 which passes through the chamber wall 17 by means of apertures 22 on opposite sides of the wall. The pin may be a solid dowel or rod, or it may be a tubular pin having a hollow center as shown in FIGS. 4 and 5. The pin has end faces 24 which are set at an angle to the axis of the pin. These end faces 24 provide pointed ends 25 on the pin in order to facilitate the insertion of the pin 23 into the holes 22 for the passage of the pins through the elongated cage 16.
Trailing end 21 of elongated cage 16 is an elongated section which contains a central bore 26 in communication with the chamber holding the cartridge 5. The bore 26 is substantially the same diameter as the bore of the chamber within the cage 16 so that a rock bolt 13 having surface ridges 14 may enter the trailing end of the cage with a loose fit. As seen in FIG. 4, when the pin has been inserted into the cage 16 the resin cartridge 5 will rest upon the top of the pin 23 and the pin will rest upon the leading end of the rock bolt 13 when the cage is oriented in a position whereby the trailing end 21 is below the leading end 19. It is to be noted that the pin prevents premature puncture of the resin cartridge by the rock bolt as the assembly is pushed down the borehole.
FIG. 5 is a sectional view of the embodiment of FIG. 4 and illustrates the various elements thereof. In particular, the pin 23 is shown with the slanted end faces 24 which provide the pointed ends 25 on the pin. Also shown are the elongated wall 17, circular perforations 18, and the cartridge 5 containing the bonding composition 6.
FIG. 6 illustates an alternate method of mounting the cage upon a rock bolt. In FIG. 6 there is shown the trailing end of the cage of FIG. 1, which comprises the elongated wall 2, the transition section 9 and an elongated section 28. The elongated section 28 has a central bore 2 which is in open communication with the chamber in which the cartridge 5 is held. The inner surface of the bore 29 is threaded with spiral threads, not shown, which are sized and adapted to mate with external threads 32 which are on the outside surface of the leading end of rock bolt 31.
Once the cage with its confined resin cartridge has been pushed all the way to the bottom of the borehole, the rock is pushed with a substantial force which is sufficient to cause the rock bolt to pass through the elongated section of the cage and penetrate the resin cartridge for the mixing and reaction of the bonding composition. In the embodiment of FIG. 1, the elongated section will expand to allow passage of the rock bolt therethrough and/or the cage 1 may fracture to allow penetration of the resin cartridge. In the embodiment of FIG. 4, the pin 23 and/or the cage 16 may fracture to allow the rock bolt to penetrate the cartridge. In the embodiment of FIG. 6, the threads on the inner surface of the bore 29 of elongated section 28 will normally shear off and/or the cage may fracture to allow penetration of the cartridge by the rock bolt.
In the device described above, in certain situations the diameter of the hole and the diameter of the bonding cartridge or similar means which needs to be used to secure the rock bolt to the walls of the hole can be very close to each other, leaving little clearance for the cage to house the bonding cartridge. This could lead to difficulties in manufacturing the product.
As an example, one size of commonly drilled hole is 25 mm in diameter, while the resin cartridge normally used in that hole size is 23 mm in diameter. The radial clearance between the hole and the bonding cartridge is only 1 mm. Allowing about 0.4 mm radial clearance for ease of insertion of the assembly into the hole (which is seldom very uniform in diameter or perfectly straight), leaves only about 0.6 mm radial clearance within which the cage must be made. Knowing that a radial clearance of at least 0.3 mm is also necessary to allow assembly of the resin cartridge or bonding means within the confines of the cage (without damaging or tearing the cartridge), it becomes necessary to ensure that the wall thickness of the cage is no more than 0.3 mm. This limitation makes it practically difficult and potentially expensive to manufacture the complete device.